Digital images are vulnerable to tampering such as cropping and replacement of image portions. Such “crop-and-replace” operations can be used to improperly add objects which were not present in the original image, or to improperly remove objects which were present in the original image. Various authentication methods have been used to detect such alterations. For example, a digital signature can be embedded in the image data as a “watermark.” Conventional digital signature watermarking techniques tend to detect any and all alterations of the original data.
However, in many cases, certain types of alterations are considered acceptable. For example, unlike crop-and-replace operations, lossy compression processing such as JPEG compression is likely to be performed for legitimate reasons such as efficiency of storage and/or transmission of the data, rather than malicious purposes such as misleading the viewer as to the presence and/or absence of objects and/or people in the scene captured by the image. Similarly, simply changing the encoding format of the image data—e.g., converting a JPEG-encoded image to a JPEG 2000-encoded image or to a GIF-encoded image—is likely to be done for innocent reasons. Likewise, filtering of an image is considered a legitimate alteration for many purposes. For cases in which lossy compression, transformation of data encoding format, and/or image filtering are considered acceptable types of image processing, conventional digital signature watermarking techniques are unsuitable, because such technologies tend to reject any data which has been altered in any manner.
Other methods have been used in order to attempt to enable acceptance of legitimate alterations. For example, in one technique, a digital signature is generated based upon selected image points which are deemed to have a particular relevance. In other techniques, digital signatures have been generated based upon moments and/or edges within an image.
However, in many cases, moment-based digital signature techniques fail to detect important malicious manipulations such as cropping and replacement. Edge-based methods suffer from excessive signature length, inconsistencies in edge detection results, and excessive sensitivity to color alteration.